1. Field of the Invention
The present invention relates generally to a technical field of a sliding member which exhibits excellent abrasion resistance as well as outstanding oxidation and corrosion resistance. More particularly, the invention relates to a sliding member in which a specific mixture rate of powder material not only improves the abrasion resistance at a high temperature but also prevents abrasion of counterpart member thereof.
2. Description of the Related Art
Technologies related to the present invention can be applied to sliding members used in a variety of parts of automobile engines such as EGR valve for exhaust gas, butterfly valve in exhaust brake or turbo charger waste gate, bearings for other rotary portions, or sliding elements in high temperature gas turbine engines. This sliding member, in particular, is effective for the use to bearings in automobiles or the like. Road sometimes becomes full of exhaust gases and a sliding surface of bearing is brought into contact with such a fluid. The sliding member is subjected to heat generation due to relative sliding motion against a sliding counterpart, and the resulting temperature increase caused by the sliding motion accelerates the abrasion of sliding surface thereof and may cause a seizure at the sliding surface. Recently, manufactures of the sliding members are urged to take an appropriate action toward the above problem. For these reasons, a sliding element is required to possess strength at a high temperature and exhibit excellent abrasion resistance. Conventional sliding members, however, fail to meet these demands since they may be able to satisfy one of the demands while they fail to satisfy the other.
Therefore, there have been increasing demands towards such a material for sliding members that enjoys outstanding corrosion and oxidation resistance as well as high abrasion resistance under a high temperature atmosphere, and is able to prevent abrasion of relatively sliding counterpart. Also the material is expected to do no harm to environments, for example, by releasing additive components of sintered parts as wear particles during the relative sliding motion. Also it is important for the sliding members to be manufactured at a low cost as a sintered machine part.
For example, there is a bearing for high temperature use as a prior art which is a sintered bearing formed by press forming, being made of stainless steel powder which is blended with distributed dry lubricant material such as carbon powder, lead monoxide or the like. The carbon powder added, however, tends to form a solid solution in the base material, stainless steel, and thereby decreases heat and corrosion resistance possessed by the original stainless steel. Also expansion of the sintered bearing due to oxidation limits the maximum working temperature under a high temperature atmosphere to less than 500° C. Lead monoxide, on the other hand, is also limited in its applications due to its potential harm to the environment.
There is another prior art which is a nickel-based bearing alloy which is also formed by sintering after blending Co—Mo—Cr—Si hard particles into Ni powder base.
However, inclusion of hard particles in the Ni powder base does enhance abrasion resistance thereof because of the hard particles, but also increases friction coefficient thereof, thereby rapidly wearing away the sliding surface of a relatively sliding rotary shaft or the like.
The inventor then developed a sintered metal bearing which includes Co powder or powder alloy containing Co in stainless steel powder in order to circumvent the above defect wherein the powder mixture is press-formed at molding pressure of 5 ton/cm2 and sintered at sintering temperature of 1150° C. for 60 minutes under a mixed atmosphere of nitrogen gas and hydrogen gas. Even this sintered metal is not free from troubles under certain conditions such as low critical seizure load under relative sliding motion in a high temperature atmosphere. When a counterpart, e.g., a relatively sliding rotary shaft, is made of free-cutting steel or the like, the hard powders contained in the bearing will wear down the counterpart.
Further, a sintered bearing containing austenitic stainless steel suffers from poor corrosion resistance compared with a bearing which is made of ingot material, due to porous structure thereof which is a characteristic nature in sintered metals. This observation can easily be verified by rapid rust formation under a salt spray test. Inconel alloy with high Ni inclusion is outstanding in this respect, but cannot be a favorable choice for bearing due to high cost thereof.
The present invention is introduced to resolve the above mentioned problems. A primary technical goal which this invention tries to achieve is to provide a sliding member which enjoys oxidation and corrosion resistance and prevents seizure on the sliding surface thereof during relative sliding motion over a wide temperature range from a low temperature to a high temperature such as 700° C. Another goal is to prevent the sliding member from causing abrasion of a relatively sliding counterpart regardless of the working temperature, high or low.